Magnetic resonance imaging (MRI) has come into very wide use for the imaging of the human body, for example, for the diagnosis of soft tissue abnormalities. MRI as currently practiced requires a large magnet capable of producing spatially uniform fields in excess of 1 Tesla, commonly fields of 1.5 Tesla and up to at least 3 Tesla. To generate such large uniform fields, superconducting electromagnets are normally employed. Commercially viable superconducting electromagnets are only superconducting at temperatures near 4 Kelvin and below. Superconducting electromagnets thus require a costly apparatus and are large and difficult to move.
MRI was an outgrowth of nuclear magnetic resonance (NMR), which is based on the following principles. Atomic nuclei, consisting of protons and neutrons, have intrinsic magnetic dipole moments which are commonly described as “nuclear spin.” In the presence of a magnetic field, spins will align preferentially along the field direction. The energy difference between the two alignments turns out to be quite small for practical fields, much smaller than that required to move an electron to an excited state or the energy of vibrational and rotational states of atoms. Spins will also rotate around the field direction at the Larmor frequency ω=γB where B is the applied magnetic field and γ is a universal constant for each nucleus; this rotation is referred to as “precession.”
An NMR analysis is carried out by placing a composition or other system to be analyzed in a high DC magnetic field and determining resonant frequencies at which electromagnetic radiation readily flips the orientation of nuclear spins. The nuclear spins whose resonant frequencies are determined may be, for example, those of the nuclei of the principal isotope of hydrogen, 1H. Those nuclei consist of a single proton.
Because the energy difference between the two nuclear spin orientations is small, the electromagnetic energy to flip the spin orientation lies in the radiofrequency range. In NMR, the resonant frequencies are used to determine properties of the composition or other system being analyzed, for example, its chemical composition. In general, the resonant frequency is equal to or very close to the Larmor frequency. The resonant frequency may differ slightly from the Larmor frequency because the nucleus “sees” a magnetic field slightly different from the applied field B due to the magnetic field being affected by the surrounding electron cloud. These differences due to the surrounding electrons, referred to as “chemical shifts,” are used to determine the identity of chemical compounds, a principal application of NMR.
When an object is placed in a high DC magnetic field and subjected to a pulse of electromagnetic energy which changes the orientation of some of its nuclear spins, the spins will gradually return back to an equilibrium configuration. The gradual return is often referred to as “relaxation” of the spins.
In MRI, an object to be imaged, for example a portion of the human body, is placed in a large uniform DC magnetic field, often called the “measurement” field. A further spatially varying magnetic field, called a “gradient” field because its magnitude generally varies linearly with position along a particular direction in the zone of interest, is also applied to the object. Radiofrequency pulses of an appropriate shape and frequency are applied which induce 1H nuclear spins in the object to align in particular directions, for example they can be rendered perpendicular to the direction of the DC magnetic field. After the radiofrequency pulse has died out, and as the spins process in the DC magnetic field, they undergo a relaxation process whereby their orientation slowly returns to collinear with the field direction, their equilibrium state. The gradient field is varied according to a predefined sequence which commonly involves the magnitude and direction of the gradient field being run through a series of discrete predetermined values which are each sustained for a period of time. The purpose of the gradient field variation sequence is to distinguish spatially (with both phase and frequency) discrete volumes of the object to be imaged. Time varying magnetic fields arising from the nuclear spin relaxation are detected. These fields are subjected to mathematical transformations, generally including a Fourier transform, which produce images of a section of the object which are used by clinicians for diagnostic purposes. The images are typically grayscale. The intensities associated with a position in the section of the object typically reflect some function of the density of 1H atoms in the immediate vicinity of that position and of the time constants for the relaxation process of the nuclear spins of those atoms.
There are many references on MRI. U.S. Pat. Nos. 4,070,611 and 4,451,788 were among the early MRI patents. U.S. Pat. No. 5,835,995 describes an MRI imaging modality of particular interest.
There is a strong interest in reducing the strength of measurement fields in MRI. Reducing equipment cost is one reason. Reduced measurement field strengths are also advantageous in that the apparatus can be made more portable if no cryogenic superconducting magnets are employed. In addition, low field measurements are less sensitive to image artifacts caused by the presence of metal implants due to the metal's susceptibility.
A reason why measurement fields in excess of 1 Tesla are employed in MRI is that the magnetic signal which is detected in MRI has a signal to noise ratio proportional to the measurement field strength. See generally, e.g., T. W. Redpath, “Signal to noise ratio in MRI,” British Journal of Radiology, 71:704-07 (1998). In order to use a lower magnetic field and yet maintain the quality of the images, it is necessary to be able to reduce the noise in the process of detection of the signal.
The normal way to detect the signal in MRI, a time-varying magnetic field, is to employ coils made of a conducting material. The coils can be made of a large number of thin conductors in parallel (“litz coils”). The time-varying magnetic field produces an electrical signal in the conducting coils as a result of Faraday's law of induction. For large fields with inductive coil detection, noise is limited by body Johnson noise. For small fields coil Johnson noise becomes significant. Johnson noise from a human body section begins to become material as compared to coil Johnson noise at measurement fields greater than a few tenths of a Tesla. Much work has gone into improving the signal to noise ratio in coil detection.
Magnetic field detectors (magnetometers) comprising superconducting quantum interference devices (SQUID) are currently regarded as the most sensitive means of measuring magnetic fields. However, SQUID-based magnetometers require very low temperatures and associated cooling equipment. Cryogenic equipment, in addition to the problems of cost, may have filling factor problems; thermal isolation requirements of sample from detector are a potential concern for signal.
It has been proposed, for example in Published U.S. Patent Application No. 20070205767 and in U.S. Pat. No. 7,145,333, to employ atomic gas magnetometers in order to detect the time varying magnetic fields which contain the signals of interest in NMR.
There are a number of difficulties with conventional atomic gas magnetometers in the MRI application. One difficulty is that some atomic gas magnetometers require an extremely low background field, whereas conventional MRI detects a small time-varying magnetic field in the presence of a large DC background magnetic field.
Thus there remains a need for magnetometers for use in MRI which introduce less noise into the measurement than conventional litz coils but which are suitable for MRI in other senses, such as for example not requiring excessive magnetic shielding, providing sufficient bandwidth, and adapting to background fields found in MRI. Such magnetometers may also have other applications, for example, to NMR.